The present invention is a multiconductor electrical cable assembly for spanning between stationary and movable machine parts on at least one of which there is an electrical device to which the cable connects. The present invention is furthermore a method of making such multiconductor cables from individual sheathed conductors or smaller cables.
Multiconductor cables which yield readily to bending forces applied in certain transverse directions and which are less flexible or yielding to forces applied orthogonally to those directions are known. A group of coplanar parallel wires joined together with a homogeneous insulating material to form a cable which is wider than it is thick is an example of a cable having these flexing properties. But cable assemblies having the right number or assortment of individual conductors with appropriate insulation or dielectric properties and the proper degrees of flexibility for a particular application often are not available. In such cases the prospective user may be obliged to tie cables together to form a makeshift assembly which does not have the desired flexing characteristics. A method of making and using a suitable cable assembly which differs from prior art practices is disclosed in detail later.
Additionally, the prior art does not suggest a multiconductor cable with a flattened central portion and end portions adjacent the central portion which are equally flexible in all directions to allow the end portions of the cable to be routed in any direction.
Finally, the known methods of joining conductors to form a cable cannot be used to form a flexible multiconductor cable assembly having a flat profile. To form such a cable, three criteria must be met. First, the joints between the conductors of the cable assembly must be able to withstand repeated flexing. Second, the joining method must not damage the sheaths of the individual conductors. Third, the flexibility of the cable must not be impaired by the bonds between the individual conductors. None of the known joining methods satisfy all of these criteria.
In heat bonding or ultrasonic bonding methods of the prior art, electromagnetic or ultrasonic energy applied to juxtaposed conductor sheaths heats adjacent portions of the sheaths to allow them to flow together into an integral structure. Energy cannot be applied to adjacent conductor sheaths to form such a bond without applying some excess energy to parts of the sheaths which do not participate in bonding. Even if the source of energy is focused only on the parts of the sheaths to be bonded, some of the energy will be conducted to adjacent areas. This excess energy can damage those adjacent areas of the sheaths. The presence of this excess energy also makes the extent of bonding difficult to limit. The flexibility of the multiconductor cable assembly is impaired if the extent of bonding is not limited. Thus, the criteria of avoiding sheath damage and maintaining cable flexibility are not met by ultrasonic or heat bonding methods.
In solvent welding methods of the prior art, a solvent for the conductor sheath material dissolves portions of juxtaposed sheaths to allow them to flow together into an integral structure. The solvent then evaporates, leaving a bond which is analogous to heat or ultrasonic bonds. Solvent welding has the same disadvantages as heat or ultrasonic bonding. The solvents typically have a low viscosity, and are thus very mobile and difficult to confine to the parts of the conductors to be fused together. Overbonding (which reduces the desired flexibility of the cable) and damage to the conductor sheaths can result.
In adhesive bonding methods a bonding material, typically having a much higher viscosity than a solvent, is introduced between the juxtaposed conductors. If necessary, the material is cured. This bonding material adheres to each conductor and coheres to itself to bond the conductors together. Adhesive bonding does not join the conductors as securely as other methods, for the bonding material does not penetrate the sheath to become an integral part thereof. The inventor has found that flexible multiconductor cable assemblies which are adhesively bonded will fail prematurely when they are repeatedly flexed in use. Adhesive bonding also stiffens the cable assembly, since the bond material added to the assembly resists flexing. Thus, adhesive bonding does not meet the criteria of withstanding repeated flexing and maintaining cable flexibility.
In mechanical bonding a series of clamps are used to bind the conductors together into a composite structure having a flat profile. This bonding method does not meet the criterion of avoiding damage to the conductor sheaths. Relative movement of the conductors, particularly when the multiconductor cable assembly is flexed, causes the conductor sheaths to chafe against the clamps and permits individual conductors to kink near the sites of clamping. The clamps can also engage other structures to interfere with the motion of a cable assembly which spans between movable and stationary machine parts.